The present invention relates to systems which combine video signals from an internal source and from an external source. Systems of this type are useful in computer controlled instruction systems and computer controlled game systems with high quality graphics. Such a system enables a combination of high quality graphics with user interaction. The high quality graphics can be derived from previously known video systems such as broadcast television, video disks or video tape recorders. These video systems are capable of generating high quality displays but are capable of only limited user interaction. On the other hand, computer controlled video graphics can be highly user interactive. However, it is extremely difficult and expensive to generate computer graphics of the same quality as is possible from the above mentioned video sources. Therefore, a combination of an externally generated video signal from a video source and a computer generated video signal appearing on the same video monitor could enable the best of each of these types.
The overlay of a computer generated video signal upon a video signal from another source involves the rapid switching from these two sources during a single horizontal line. It is understood by those skilled in the art that the two video sources must be properly synchronized both in horizontal lines and in vertical frames in order to minimize the flicker or movement of images on the video monitor. Only if the two video sources are phase synchronized will the picture on the video monitor be free from flicker, rolling or distorted images.
In the field of the present invention, a computer generated internal video signal is overlaid upon an external video signal coming from a source not under control of the computer. In such a case, the computer generated signal must be synchronized to the external signal. This task is relatively easy if a perfect external video signal is received. However, problems occur when the external signal is less than perfect, that is when it includes time base errors, possible signal loss and possible color loss. in such an event, it is difficult to cause the internally generated video signal to follow the external signal in order to provide a stable display at the video monitor.